The
meninges are three layers of protective tissue called the
dura
mater,
arachnoid
mater, and
pia
mater that
surround the neuraxis. The meninges of the brain and spinal
cord are continuous, being linked through the magnum
foramen.

Dura Mater

The
dura mater is the most superior of the meningeal
layers. Its name means "hard mother" in Latin and it is
tough and inflexible. This tissue forms several structures
that separate the cranial cavity into compartments and
protect the brain from displacement.

The
falx
cerebri
separates the hemispheres of the cerebrum.

The
falx
cerebelli
separates the lobes of the cerebellum.

The
tentorium
cerebelli
separates the cerebrum from the cerebellum.

The dura mater
also forms several vein-like sinuses that carry blood (which
has already given its supply of oxygen and nutrients to the
brain) back to the heart.

The
superior
sagittal sinus runs across the top of the brain in
an anterior-posterior direction.

Other sinuses
include the straight sinus, the inferior sinus, and the
transverse sinus.

The
epidural
space is a
potential
space between the dura mater and the skull. If there is
hemorrhaging in the brain, blood may collect here. Adults
are more likely than children to bleed here as a result of closed head injury.

The
subdural
space is
another potential space. It is between the
dura mater and the middle layer of the meninges, the
arachnoid mater. When bleeding occurs in the cranium, blood
may collect here and push down on the lower layers of the
meninges. If bleeding continues, brain damage will result
from this pressure. Children are especially likely to have
bleeding in the subdural space in cases of head
injury.

Arachnoid
Mater

The
arachnoid or arachnoid mater is the middle layer of the
meninges. In some areas, it projects into the sinuses formed
by the dura mater. These projections are the arachnoid
granulation/arachnoid villi. They transfer cerebrospinal fluid from
the ventricles back into the
bloodstream.

The
subarachanoid space lies between the arachnoid
and pia mater. It is filled with cerebrospinal fluid. All
blood vessels entering the brain, as well as cranial nerves
pass through this space. The term arachnoid refers to the
spider web like appearance of the blood vessels within the
space.

Pia
Mater

The
pia
mater is
the innermost layer of the meninges. Unlike the other
layers, this tissue adheres closely to the brain, running
down into the sulci and fissures of the cortex. It fuses
with the ependyma, the membranous lining of the
ventricles to form structures called the choroid
plexes
which produce cerebrospinal fluid.

Cerebrospinal
Fluid

Purpose

Cerebrospinal fluid is a clear liquid
produced within spaces in the brain called ventricles. Like saliva it is a filtrate of blood. It is
also found inside the subarachnoid space of the meninges
which surrounds both the brain and the spinal chord. In
addition, a space inside the spinal chord called the central
canal also contains cerebrospinal fluid.

It acts as a
cushion for the neuraxis, also bringing nutrients to the
brain and spinal cord and removing waste from the
system.

Choroid
Plexus

All of
the ventricles contain choroid plexuses which produce
cerebrospinal fluid by allowing certain components of blood
to enter the ventricles. The choroid plexuses are formed by
the fusion of the pia mater, the most internal layer of the
meninges and the ependyma, the lining of the
ventricles.

The Ventricles

These
four spaces are filled with cerebrospinal fluid and protect
the brain by cushioning it and supporting its weight.

The
two
lateral ventricles extend across a large area of the
brain. The anterior horns of these structures are located in
the frontal lobes. They extend posteriorly into the parietal
lobes and their inferior horns are found in the temporal
lobes.

The
third
ventricle lies between the two thalamic
bodies. The massa intermedia passes through it and the
hypothalamus forms its floor and part of its lateral walls.

The
fourth
ventricle is located between the cerebellum
and the pons.

The four
ventricles are connected to one another.

The two
foramina of
Munro, which are also know as the
interventricular
foramina, link the lateral ventricles to the
third ventricle.

The
Aqueduct of
Sylvius which is also called the cerebral
aqueduct connects the third and fourth ventricles.

The fourth
ventricle is connected to the subarachnoid space via two
lateral foramina of
Luschka and by one medial foramen of
Magendie.

Subarachnoid Space

Although cerebrospinal fluid is
manufactured in all of the ventricles, it circulates through
the system in a specific pattern, moving from the lateral
ventricle to the third, and then from the third to the
fourth. From the fourth ventricle, the cerebrospinal fluid
passes into the subarachnoid space where it circulates
around the outside of the brain and spinal cord and
eventually makes its way to the superior sagittal sinus via
the arachnoid granulations also called arachnoid villi. In the
superior sagittal sinus, the cerebrospinal fluid is
reabsorbed into the blood stream.

The cerebrospinal
fluid of the neuraxis is regenerated several times every
twenty-four hours.

Endolymph and perilymph, the fluids of the inner
ear, are derived from cerebrospinal fluid. Currently, there
is no consensus regarding the manner in which cerebrospinal
fluid enters the inner ear. Osmosis may be involved.

A condition called
hydrocephalus occurs when, for some
reason, too much cerebrospinal fluid is produced and the
ventricles swell, putting pressure on the tissue of the
brain. Tumors are one potential cause of an over-production
of cerebrospinal fluid.

Hydrocephalus
should not be confused with hydroencephali. The term hydroencephali
literally means "water brain" and refers to a rare birth
defect in which the cerebrum is absent and the space where
it should be is entirely filled with cerebrospinal fluid.

In the past,
before CT and MRI technology existed, a technique involving
cerebrospinal fluid called pneumoencephalography was used to view the
brain. A small amount of cerebrospinal fluid was removed
from the ventricular system and replaced with air or some
other inert gas. This allowed the examiner to view the
ventricles in a scan and make inferences about brain
pathology. Tumors and hemorrhages could sometimes be located
by examining the shapes and sizes of the ventricles. Because
space within the cranium is limited, growths or coagulated
blood (hematoma) will displace white and gray matter,
pushing them into the ventricular system.

Cerebrospinal
fluid can be analyzed to make judgments about a person's
general health as can blood and saliva .A sample is taken from the spinal cord via a
lumbar
puncture
which is also known as a spinal tap.